1. The Field of the Invention
The present invention relates to the production of hydrogen from anaerobic digestion. In particular, the present invention relates to methods for enriching biomass with hydrogen-producing bacteria or spores and methods of producing hydrogen using the enriched biomass.
2. The Related Technology
Hydrogen is a potentially good fuel source for power generation and is currently used in many important industrial applications such as the manufacture of fertilizers. Although hydrogen is typically not used as a fuel source, hydrogen is often thought of as an environmentally superior fuel to hydrocarbons because when hydrogen is burned, it reacts with oxygen to produce environmentally harmless water. Unfortunately, hydrogen does not exist in nature in useable quantities; consequently, it has to be manufactured. Most hydrogen produced today is manufactured from hydrocarbons such as petroleum or natural gas. The disadvantage of manufacturing hydrogen from these is that they are expensive and non-renewable energy sources. Furthermore, hydrogen manufacturing using petroleum or natural gas consumes substantial amounts of energy and/or requires expensive catalysts, such as platinum based catalysts.
Anaerobic digestion provides a potentially improved alternative to manufacturing hydrogen from petroleum and natural gas. Anaerobic digesters can produce hydrogen from inexpensive and renewable energy sources such as organic wastes (e.g. food processing waste and animal waste). Recent studies have shown that certain strains of bacteria (e.g. bacteria from the genus Clostridium) are particularly effective at producing hydrogen as a by-product during anaerobic digestion of organic waste material.
One problem with digesting organic waste in an anaerobic digester is that organic waste such as manure includes naturally occurring bacteria. Many of these bacteria consume hydrogen. Eventually, an anaerobic digester fed with non-sterile material will create a bacterial culture that is a mixture of competing bacteria, some of which consume hydrogen. Absent some intervention, hydrogen-consuming bacteria will invariably grow until most or all of the hydrogen being produced is simultaneously consumed.
Several systems have been developed to allow hydrogen to be produced in an anaerobic digester. These systems typically require growing and maintaining pure strains of hydrogen-producing bacteria and sterilizing the material to be digested. These systems are not commercially viable because maintaining a pure strain of bacteria in a digester is difficult and sterilizing the material to be digested is very expensive.
Recently, an improved method has been developed for obtaining quantities of hydrogen-producing bacteria. In this method, a mixed culture of bacteria is heat treated to destroy the hydrogen-consuming bacteria. The hydrogen-producing bacteria survive the heat treatment by creating spores. Thus the treated culture is enriched with hydrogen-producing bacteria as compared to hydrogen-consuming bacteria. The enriched culture is then used to seed an anaerobic digester.
One problem with forming a seed culture using existing methods is that it requires an expensive heat treatment step. U.S. Pat. No. 6,860,996 to Noike et al, teaches reducing the cost of forming a seed culture by heat treating at temperatures between 68° C. and 95° C. While the Noike process may reduce costs, heat treating to above 68° C. is still expensive. Furthermore, Noike teaches sterilizing the material to be digested, which is also expensive.
Therefore, what is needed is a system for anaerobically digesting organic materials that is more cost effective and more efficient than existing systems.